Many people start using calculators at a young age, and more so when they get older. They use it to do math at school, and they’ll eventually use it to manage their finances. With so many calculator users around the world, it’s important that a calculator’s user interface is easy to use for both younger and older users. However, with the way calculators work today, it’s not that easy to use for users at any age.

Hidden Operator States

The biggest problem with calculator user interfaces is that they don’t show you what operator state it’s in. Your calculator could be in an operator state, but you won’t know which one because the calculator screen in different operator states look exactly the same.

Screen doesn’t show operator states.

For example, on a calculator screen with the number ‘1’, a user could press the number ‘1’ again and depending on what state it’s in, it’ll either display the new number as ‘1.1’, ‘1’ or ‘11’. The user doesn’t know if it’s in a decimal, addition, subtraction, multiplication, division state or no state at all.

The user has to cognitively work harder to remember which operator they pressed, and where they are in their operation. This is not only difficult for users who have working memory problems, but also for users who calculate large operations with multiple operators, or those who punch in calculator buttons so fast that they can easily forget which operator button they last pressed.

A better calculator user interface won’t force users to remember what operator state they’re in. It also won’t force users who often punch in buttons fast, to start punching them in slower for the fear of making a mistake.

Recalling Previous Operands

Hidden operator states aren’t the only problem with calculators today. Another problem users have is recalling previous operands in their operation. This is necessary for users who are calculating large operations with same operands that appear multiple times.

Screen doesn’t show operation progress.

For example, if a user is trying to calculate the operation ‘15+25+25+35+45+55+25+25+35+45+55+25’, it’ll be difficult for them to keep track where they are in the middle of the operation because so many of the same operands repeat themselves. Seeing ‘25’ on their screen doesn’t tell them exactly where they are in the operation because ’25’ comes up multiple times. It could be the ‘25’ at the beginning, middle or end of the operation.

A better calculator user interface would show users the previous operands in their operation. This way they’ll know exactly where they are when identical operands appear multiple times.

Correcting Wrong Operands

Another common problem that users have with calculators is entering the wrong operands and not being able to correct them. With the way calculators work today, users can only correct the current operand they’re entering or clear the entire operation. They can’t target and fix a specific operand or operator they previously entered.

Users have to clear all to correct previous operands.

This is time-consuming for users who are calculating large operations because it forces them to have to clear their entire operation, start over and enter it perfectly. Or, they have to offset their errors by adding more operands and operators to their operation. But this can confuse users and make them unsure whether their actions will correctly affect their result.

A better calculator user interface would allow users to go back and correct any wrong operator or operand they’ve entered at any time. Users won’t have to reset and start over when they make a mistake.

Unclear Clear Buttons

It’s not just the calculator screen that users have issues with. The clear buttons on calculators also give users trouble too. Some calculators have a ‘CE’ and ‘C’ button. The ‘CE’ stands for ‘clear entry’; it clears the current operand on the screen. The ‘C’ button stands for ‘clear’; it clears the entire operation.

Similar look confuses users.

These two buttons look and function so similarly that most users aren’t sure what their differences are. This uncertainty causes them to push both buttons to make sure that their previous operation has cleared and that it’s safe to start a new one. Some users even hit the buttons multiple times because they feel so unsure about it.

The only thing that’s clear about calculator clear buttons is that they confuse users and cause them to do a lot of unnecessary work. A better calculator user interface will assure users that their operation has cleared after pressing the clear button once. It’ll also prevent users from hitting the wrong clear button.

A Better Calculator User Interface

An excellent way to solve the problem of hidden operators and recalling previous operands is to display a string of the user’s current operation in small font at the top of the screen. The user’s operation would populate from left to right and display both operands and operators. If the user turns on an operator, the operator symbol will inverse highlight with white font on a small black box clearly showing what operator state they’re in.

Users see current operator state, previous operators and operands.

What’s great about this approach is that the user can get interrupted, walk away from their calculator, come back and pick up where they left off without starting over or remembering a thing. The user can see a history of everything they have done in the top operation string and easily continue with their calculation.

Users can move cursor to previous operators and operands and correct them.

To allow users to correct any operand or operator, the small black box that highlights the current operator will also act as a cursor that users can move left and right. Users can move the cursor to any operand or operator and directly enter a new operand or operator over the wrong one. The cursor also allows users to view their entire operation no matter how long it gets.

With this new function, the calculator will need a left and right arrow button. The buttons would be placed away from the clear buttons, and use arrow heads only so that they don’t visually interfere with the arrows on the delete buttons.

New clear entry icon looks less like clear button and more like backspace.

To make the calculator clear buttons less confusing, the clear (C) and clear entry (CE) button will look nothing like each other. Only the clear button will have the letter ‘C’ so that users know that there is only one clear button. The CE button will instead look more like the backspace button because they both function similarly. The backspace button deletes one digit at a time, while the CE button deletes all the digits in the user’s current operand. To distinguish the CE button from the backspace button, it’ll have an arrow pointing to a vertical bar, which symbolizes deletion to a stronger degree.

This new icon will help users intuitively understand the difference between the delete buttons. It’ll also prevent users from mistaking the clear entry button for the clear button. Users can press the clear button just once safely know that their operation has cleared.

Something that hasn’t changed in this user interface is the user’s current operand area. This primary status area shouldn’t change because it conveniently shows users what numbers they’re entering, and what the calculation results are in a clear and fixed place that they can easily find. Showing numbers here in larger font keeps the smaller numbers in operation string above it separate so that users won’t ever confuse the two.

This solution solves all the big problems that users have with calculators today. But one problem it doesn’t solve is whether more calculator interfaces will start using this design.

I also immediately thought TI when I read this post. I was at a Open Hardware conference a few weeks ago and a college student pulled out a TI-85 – I had one back in 1997! Couldn’t believe how little had changed.

I think I still have my HP48GX calculator in a box in the attic somewhere. That already nailed it. Everything you mention, plus RPN stack based if you wanted/got that, symbolic expression operations, graphics, etc. It’s been done already, many years ago…